In order to improve the signal-to-noise ratio and the sensitivity of photodetectors, it is important to limit dark current output. The detection of long wavelength and low background infrared radiation is particularly difficult with conventional multiquantum well photodetectors due to the presence of a relatively large dark current. High dark currents also limit the dynamic range and increase the power dissipation of these devices. In the fabrication of focal plane arrays, power dissipation is a major concern due to the need for additional heat dissipation control.
Semiconductor photodetectors having multiple quantum wells in a superlattice structure are known in the art. A superlattice is typically fabricated by molecular beam epitaxy or metalorganic chemical vapor deposition techniques to form a multilayered heterojunction structure. The thickness of each active layer is reduced to the order of carrier de Broglie wavelength such that a series of discrete energy levels are produced, A typical superlattice photodetector may include a plurality of gallium arsenide (GaAs) layers alternating with aluminum gallium arsenide (AlGaAs) layers. Each period of the superlattice thus comprises one GaAs layer and one AlGaAs layer. The GaAs layers are heavily doped n-type to form the quantum well layers which are positioned between the AlGaAs barrier layers. The conduction band edge of the barrier layer material is above the conduction band edge of the quantum well layers such that the quantum wells are periodic. It is know that the height of the energy barriers of the barrier layers can be controlled by adjusting the mole fraction of aluminum to confine electrons at selected energy levels in the quantum wells. In order to reduce thermionic emission of electrons from the quantum wells, these devices are operated at low temperatures according to the detection wavelength.
In operation, an electrical bias is applied perpendicular to the alternating barrier and quantum well layers. In the absence of illumination, this produces a current known as the dark current. Dark current results primarily from quantum mechanical tunneling of electrons through the potential barriers of the barrier layers. Upon illumination by photons of the appropriate energy, electrons are excited out of the quantum wells by transitions between energy levels. The photoemission of electrons from the quantum wells increases the conductivity of the device. Thus, it will be appreciated that these devices can be used to generate a signal which is proportional to the magnitude of the detected radiation.
One such device is disclosed in European Patent No. 275-150-A, wherein a photodetector having a superlattice is provided for infrared radiation detection. Electrons in the quantum wells have two bound states. Incident infrared radiation produces intersub-band absorption between the ground state and the excited state. The applied bias, the height of the potential energy barriers of the barrier layers, and the spacing of the energy states in the quantum well layers are set such that electrons in the excited state have a high tunneling probability. A signal is produced by tunneling of the photoexcited electrons through the potential barriers of the barrier layers. In one embodiment, energy levels of neighboring wells are matched to optimize tunneling of photoexcited electrons while inhibiting dark current tunneling.
Other superlattice photodetectors have been designed which do not rely on photoexcited tunneling for the signal current. These devices are based on the recognition that quantum well structures have finite barrier heights and that permissible energy states exist above the potential barrier of the barrier layers, i.e., in the continuum state of the superlattice. B. F. Levine and others described a photodetector of this type in "High-Detectivity D*=10.sup.10 cm, .sqroot.Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector," Appl.Phys.Lett., 53(4), 25 July 1988. The detector is a 50 period GaAs/AlGaAs superlattice grown on a semi-insulating GaAs substrate which is sandwiched between contact layers. One advantage of this type of device is its ability to control peak absorption wavelength. This is achieved by varying the dimensions of the quantum well layers and the barrier layers. The quantum wells contain a single bound state. Through photoemission of quantum well electrons into the continuum state while the superlattice is biased, electrons travel above the potential barriers toward the collector contact rather than through the barriers by quantum mechanical tunneling. Assuming an adequate mean-free path, the photoexcited carriers produce a signal representative of photon absorption in the quantum well layers.
Although multiple quantum well structures provide higher absorption efficiencies than single-well devices, a larger bias voltage is also required. This, in turn, increases the dark current produced by conventional superlattice photodetectors. Although the thermionic emission component of the dark current can be effectively minimized by operating at low temperatures, the tunneling current, which is increased by sequential resonant effects and electron hopping, produces a significant dark current. As stated, in applications requiring the detection of long wavelength and low background infrared radiation, the dark current is a major problem in the operation of conventional multiquantum well photodetectors. Therefore, it is important to reduce the tunneling component of the dark current.
One solution proposed by others to reduce dark current is to increase the thickness of each of the individual barrier layers of the superlattice. Since photoconduction is not achieved through tunneling, thin barriers are not necessary from the standpoint of optimizing tunneling current. More specifically, in the aforementioned photodetector described by Levine and others, 300 angstrom AlGaAs barrier layers and 40 angstrom GaAs quantum well layers were arranged to form a 50 period superlattice. By increasing the barrier width from 140 to 300 angstroms and the barrier height from 160 mV to 250 mV, the dark current was reduced by several orders of magnitude. This reduction in dark current resulted from a decrease in electron tunneling through the thick barrier layers. However, this method of decreasing the dark current suffers from several serious limitations.
Photodetector performance is based primarily on quantum efficiency, response time and sensitivity. Although increasing the thickness of the superlattice barrier layers reduces dark current, this limits the quantum efficiency of the detector. It will be appreciated that the barrier layers represent the low-mobility portion of the superlattice structure. Thus, if barrier layer thicknesses are substantially increased to reduce the dark current, electron mobility through the superlattice structure is decreased.
The high potential barrier of the proposed thick barrier layers also produces undesirable effects. More specifically, as the separation between the continuum state and the barrier height decreases, scattering of the photoexcited electrons increases in the barrier region, which reduces carrier mobility. In addition, as the barrier height approaches the energy level of the continuum state, some photoexcited electrons flow in opposition to the applied field. As a result, the signal-to-noise ratio decreases due to the increase in noise and the reduction in the number of electrons which reach the collector contact.
It is also known that radiation hardness is often an important characteristic of multiquantum well superlattice infra red detectors. More specifically, radiation-hard devices are required in space applications due to the high levels of gamma and other ionizing radiation.
In addition to the foregoing work by others, in U.S. Pat. No. 4,645,707, a semiconductor device is disclosed which includes two superlattices separated by a centrally disposed barrier layer which has a lower transmission coefficient than the barrier layers of the superlattices. It is stated that the central barrier layer is thicker than the barrier layers of the superlattices. The semiconductor device exhibits negative differential conductance due to voltage dependent discontinuities between energy minibands of the two superlattices. The device is a tunneling current device and is not a photodetector.
In contrast to the prior art suggestion of increasing the thicknesses of the superlattice barrier layers, in U.S. Patent Application Ser. No. 457,613, filed Dec. 27, 1989, now U.S. Pat. No. 5,077,593 issued on Dec. 31, 1991, which was assigned to the assignee of the present invention and which is incorporated herein by reference, the inventors of the present invention disclose a multiquantum well superlattice infrared detector in which tunneling current is prevented by a single, thick blocking layer positioned between the superlattice and the collector contact. More specifically, the photodetector disclosed therein includes a plurality of alternating quantum well layers and barrier layers arranged to form a superlattice. A blocking layer of barrier material is provided between the superlattice and the collector contact. The blocking layer thickness is selected such that it substantially eliminates the tunneling current component of the photodetector dark current. The blocking layer is substantially thicker than the individual barrier layers of the superlattice. Electrons tunneling through the barrier layers which would otherwise contribute to the dark current of the photodetector are blocked by the presence of the thick blocking layer. By blocking tunneling current in this manner through the use of a single blocking layer, dark current is reduced without compromising quantum efficiency.
In order to further enhance the performance of the aforementioned end-blocked multiquantum well superlattice infrared detector and other multiquantum well superlattice detectors, the present invention provides several improvements which increase the quantum efficiency and decrease the generation of noise in these devices.
One object of this invention is to provide several techniques for improving the performance of multiquantum well superlattice infrared detectors generally.
Another object of the present invention is to provide a low dark current end-blocked multiquantum well photodetector in which electron transport in the continuum energy level is optimized.
Still another object of the present invention is to provide an improved end-blocked multiquantum well photodetector having a low dark current in which quantum efficiency is improved by the use of a materials system which is superior to GaAs/AlGaAs.
It is still a further object of the present invention to provide an end-blocked multiquantum well photodetector which is particularly efficient in the detection of long wavelength and low background infrared radiation with virtually no power dissipation.
Still another object of the present invention is to provide an improved, low dark current, end-blocked multiquantum well photodetector for use in infrared detector focal plane arrays.